Color Highlights with Box Hair Color
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A: You need to remember that the box hair color kits are designed most commonly for singly colored hair. If you look on the packaging, there is usually a chart showing the expected color results based on your hair's natural shade. Even when using darker hair color formulas, these hair colors are designed to add color to the existing hair color.
There is also the fact that the hair that was lightened excessively may be more porous than the rest of the hair and the hair color you add may fade more quickly from these areas.
The good news is that if the hair is healthy and in good condition, you can simply add more color until you get the results you want. However, I do suggest that when you recolor, that you add the color to the lighter hair first, wait ten minutes then add the color to the remainder of the hair. This will allow the lighter hair to absorb more color and ultimately balance out the discrepancy in color shades.
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See also:
How to dye hair
Dye over previous hair color with box dye
What the numbers on the hair dye boxes mean